The finite element analysis of stresses in atherosclerotic arteries during balloon angioplasty

Computational Assessment of Stent Durability Using Fatigue to Fracture Approach

Stents are metal scaffold devices used to maintain lumen and restore blood flow of diseased artery. Despite they brought care of coronary diseases to a new level of efficacy, problem of stent fracture remains unclear even after global needs reached number of 5 × 106 devices yearly. For projected work-life of 10 years, rate of fracture occurrence in stents varies from 5% up to 25% for different designs. Analysis of such miniature devices and long-term events in realistic in vivo conditions remains impossible while experimental in vitro measurements provide limited results consuming much time and expensive equipment. The principal aim of this study was to propose procedure for numerical estimation of coronary stents durability assuming the hyperphysiological pulsatile pressure conditions. The hypothesis was whether the stent durability would be achieved safely for the projected work-life of 10 yr? The procedure was carried out within three phases: (a) initial fatigue analysis based on S-N approach; (b) fatigue lifetime assessment based on fatigue crack growth simulation using Paris power law, and (c) safe-operation, i.e., no-fatigue failure (based on Kitagawa–Takahashi diagram) as well as immediate predictions of the fracture event in the stent. For considered generic stent design, results showed that the stent durability would be achieved safely. Since special diagrams were used, the fatigue risk assessment was clearer compared to the conventional fatigue lifetimes. Moreover, it was found that crack growth was stable for both small and large scale sizes of the crack. Besides the fact that the presented procedure was shown as suitable for numerical assessment of the generic stent durability under hyperphysiological pulsatile pressure conditions, it was concluded that it might be applied for any other design as well as loading conditions. Moreover, it could be efficiently combined with experimental procedures during the process of the stent design validation to reduce manufacturing and testing costs.

A novel arterial constitutive model in a commercial finite element package: Application to balloon angioplasty

Recently, a novel linearized constitutive model with a new strain measure that absorbs the material nonlinearity was validated for arteries. In this study, the linearized arterial stress-strain relationship is implemented into a finite element method package, ANSYS, via the user subroutine USERMAT. The reference configuration is chosen to be the closed cylindrical tube (no-load state) rather than the open sector (zero-stress state). The residual strain is taken into account by analytic calculation and the incompressibility condition is enforced with Lagrange penalty method. Axisymmetric finite element analyses are conducted to demonstrate potential applications of this approach in a complex boundary value problem where angioplasty balloon interacts with the vessel wall. The model predictions of transmural circumferential and compressive radial stress distributions were also validated against an exponential-type Fung model, and the mean error was found to be within 6%.

Simulation of stent deployment in a realistic human coronary artery

Background

The process of restenosis after a stenting procedure is related to local biomechanical environment. Arterial wall stresses caused by the interaction of the stent with the vascular wall and possibly stress induced stent strut fracture are two important parameters. The knowledge of these parameters after stent deployment in a patient derived 3D reconstruction of a diseased coronary artery might give insights in the understanding of the process of restenosis.

Methods

3D reconstruction of a mildly stenosed coronary artery was carried out based on a combination of biplane angiography and intravascular ultrasound. Finite element method computations were performed to simulate the deployment of a stent inside the reconstructed coronary artery model at inflation pressure of 1.0 MPa. Strut thickness of the stent was varied to investigate stresses in the stent and the vessel wall.

Results

Deformed configurations, pressure-lumen area relationship and stress distribution in the arterial wall and stent struts were studied. The simulations show how the stent pushes the arterial wall towards the outside allowing the expansion of the occluded artery. Higher stresses in the arterial wall are present behind the stent struts and in regions where the arterial wall was thin. Values of 200 MPa for the peak stresses in the stent strut were detected near the connecting parts between the stent struts, and they were only just below the fatigue stress. Decreasing strut thickness might reduce arterial damage without increasing stresses in the struts significantly.

Conclusion

The method presented in this paper can be used to predict stresses in the stent struts and the vessel wall, and thus evaluate whether a specific stent design is optimal for a specific patient.

A nonlinear, anisotropic and axisymmetric model for balloon angioplasty

A nonlinear elastic, anisotropic and axisymmetric balloon angioplasty model, consisting of the balloon, the atherosclerotic plaque and the artery wall, has been developed and analysed in this paper. The deformation of the angioplasty compound, i.e. the balloon, the plaque and the artery, for slowly increasing dilation pressure within the balloon, is investigated. The plaque has been considered as a circular cylindrical tube extending all around the artery circumference. Normal radial stress and radial displacement boundary conditions have been imposed along the balloon–plaque and the plaque–artery interfaces. Large elastic deformations have been accommodated in the model. The balloon material has been considered as isotropic, the plaque material as transversely isotropic and the artery wall material as orthotropic. A strain energy density function, that satisfies existing incomplete experimental data, has been constructed for the plaque material. For a given dilation pressure within the balloon, the deformation of the angioplasty compound is determined via the numerical solution of a system of four nonlinear equations. For a medium hard balloon and a medium stiff plaque, a dilation pressure 7.5 times bigger than the blood pressure is required for an almost full reopening of the narrowed lumen. The percentage increase in length of the inner radius of the plaque is twice bigger than that of the radius of the artery. Results also indicate that the process of full reopening may be achievable, without stressing the artery wall dramatically.

Finite element modelling and simulations in cardiovascular mechanics and cardiology: A bibliography 1993–2004

The paper gives a bibliographical review of the finite element modelling and simulations in cardiovascular mechanics and cardiology from the theoretical as well as practical points of views. The bibliography lists references to papers, conference proceedings and theses/dissertations that were published between 1993 and 2004. At the end of this paper, more than 890 references are given dealing with subjects as: Cardiovascular soft tissue modelling; material properties; mechanisms of cardiovascular components; blood flow; artificial components; cardiac diseases examination; surgery; and other topics.

A predictive study of the mechanical behaviour of coronary stents by computer modelling

Intravascular stents are small tube-like structures expanded into stenotic arteries to restore blood flow perfusion to the downstream tissues. The stent expansion is an important factor to define the effectiveness of the surgical procedure: it depends on the stent geometry and includes large displacements and deformations, geometric and material non-linearity. Numerical analyses seem appropriate to study such a complex behaviour after a free stent expansion. In this study the finite element method (FEM) was applied to a new generation coronary stent. Results from computations were compared with those from a laboratory experiment in terms of radial expansion and elastic recoil. By means of a scanning electronic microscopy the area of plastic deformation were also detected and compared with those obtained in the numerical simulation. Matching between the different measurements was quite satisfactory even if some discrepancies were present due to the absence of the balloon in the numerical model.

Blood vessel constitutive models-1995-2002

Knowledge of blood vessel mechanical properties is fundamental to the understanding of vascular function in health and disease. Analytic results can help physicians in the clinic, both in designing and in choosing appropriate therapies. Understanding the mechanical response of blood vessels to physiologic loads is necessary before ideal therapeutic solutions can be realized. For this reason, blood vessel constitutive models are needed. This article provides a critical review of recent blood vessel constitutive models, starting with a brief overview of the structure and function of arteries and veins, followed by a discussion of experimental techniques used in the characterization of material properties. Current models are classified by type, including pseudoelastic, randomly elastic, poroelastic, and viscoelastic. Comparisons are presented between the various models and existing experimental data. Applications of blood vessel constitutive models are also briefly presented, followed by the identification of future directions in research.